1. Field of the Invention
This invention relates to amplifiers in general and more particularly to amplifiers for use with weak signals for radio frequency (RF) and audio frequency applications. The invention is ideally suited for use in radio and television receivers. However, it will be appreciated that the invention can be used in any electronic device where weak signals are to be amplified and signal gain is to be increased.
2. Description of the Prior Art
Radio receivers have been in existence for almost a century. The earliest receivers were extremely crude in design and consisted of no more than a crystal receiver with a simple diode connected to a pair of headphones. The diode and headphone combination acted as a detector/low pass filter wherein an RF signal generated by the crystal was detected by the diode and the low frequency aspect was passed by the headphones while the higher frequency carrier was blocked. These circuits included a so-called "cat's whisker" wherein a wire comprising the cat's whisker would be placed against various spots in the crystal detector. The process of moving the cat's whisker from one location to another was a crude form of station tuning whereby the frequency at which the crystal was oscillating was essentially shifted slightly thereby causing the crystal to act as a detector/demodulator for that given frequency. Thus, the signal was effectively demodulated and the audio portion could be heard in the headphones.
This crystal detector type of radio suffered from many disadvantages. Among those were a lack of sensitivity, poor station selection and low power. The low output power was witnessed by the fact that only a few stations could be detected by the radio.
To overcome these disadvantages, tuned radio frequency (TRF) receivers were developed. TRF receivers gave improved selectivity, sensitivity and output power by adding radio frequency and audio frequency amplifier stages. The purpose of TRF receivers was essentially to enhance signal gain and the result was that more stations could be detected and heard over speakers as well as with headphones.
A significant problem with this type of receiver was distortion. This distortion resulted from the occurrence of positive feedback between amplifier stages. The distortion problem was particularly acute at higher frequencies. Higher frequencies were more difficult for the receiver to process due to the fact that the various leads comprising the receiver signal processing stages would act as "mini antennae" radiating the signal into all parts of the receiver and thereby introducing feedback and unwanted distortion into the circuit.
In addition, physical coupling (conduction) of the RF signals from one stage to another occurs. For example, the common power supply bus requires filtering devices between stages to attempt to reduce the coupling of the RF signals from one stage to another through the power supply distribution bus line. Therefore, as used herein, the term "radiating", or its equivalent, is intended to include "conduction" of the signal by a physical coupling such as a common power supply bus.
The next major advancement was the superheterodyne receiver. The superheterodyne has many important advantages over the above receiver types. The main distinction between the superheterodyne and the earlier versions of radio receivers is that the superheterodyne will amplify an RF signal in at least two different stages before it reaches the audio amplifier. The method with which this occurs represents a major improvement over previous designs. In a superheterodyne there are a minimum of two amplifier stages. A first stage includes an RF amplifier for providing gain to the weak radio frequency signals received from the antenna. A mixer stage, or single conversion stage, mixes the amplified RF signal with an oscillator signal. These two signals are designed to differ by a specified frequency. The resultant output of the first detector stage is called the intermediate frequency ("IF") and represents the difference between the oscillator frequency and the RF frequency. This intermediate frequency is then fed through an IF amplifier and a detector wherein the high frequency component is eliminated and the remaining audio signal is then fed to an audio amplifier. The purpose for using an IF stage is to reduce the RF frequency in the first stage to a second frequency different from the RF frequency so that radiation from the RF circuit would not adversely influence or be coupled to the IF stage during amplification in the IF stage. In addition, amplification in the IF stage would not affect the RF stage. Thus the superheterodyne receiver allowed more amplification and gain in the receiver since the two stages, operating at different frequencies, did not adversely influence each other.
The problem of feedback and oscillation continues to exist when amplifier stages operating at the same frequency are in the vicinity of each other. This is, again, due to signal radiation from one amplifier stage to another. This places a severe limitation on the abilities of amplifiers to increase the output power of a signal. The problem is even more pronounced at higher frequencies where individual portions of the circuit, such as connection terminals, will act as antennae thus feeding the output signals back into the input portions.